pharmaceutics Article Design and Characterization of Inulin Conjugate for Improved Intracellular and Targeted Delivery of Pyrazinoic Acid to Monocytes Franklin Afinjuomo 1, Thomas G. Barclay 1, Ankit Parikh 1, Yunmei Song 1, Rosa Chung 1, Lixin Wang 1, Liang Liu 1, John D. Hayball 1, Nikolai Petrovsky 2,3 and Sanjay Garg 1,* 1 School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, SA 5001, Australia; olumide.afi[email protected] (F.A.); [email protected] (T.G.B.); [email protected] (A.P.); [email protected] (Y.S.); [email protected] (R.C.); [email protected] (L.W.); [email protected] (L.L.); [email protected] (J.D.H.) 2 Vaxine Pty. Ltd., Adelaide, SA 5042, Australia; nikolai.petrovsky@flinders.edu.au 3 Department of Endocrinology, Flinders University, Adelaide, SA 5042, Australia * Correspondence: [email protected]; Tel.: +61-8-8302-1567 Received: 26 April 2019; Accepted: 16 May 2019; Published: 22 May 2019 Abstract: The propensity of monocytes to migrate into sites of mycobacterium tuberculosis (TB) infection and then become infected themselves makes them potential targets for delivery of drugs intracellularly to the tubercle bacilli reservoir. Conventional TB drugs are less effective because of poor intracellular delivery to this bacterial sanctuary. This study highlights the potential of using semicrystalline delta inulin particles that are readily internalised by monocytes for a monocyte-based drug delivery system. Pyrazinoic acid was successfully attached covalently to the delta inulin particles via a labile linker. The formation of new conjugate and amide bond was confirmed using zeta potential, Proton Nuclear Magnetic Resonance (1HNMR) and Fourier transform infrared spectroscopy (FTIR). Scanning electron microscopy (SEM) confirmed that no significant change in size after conjugation which is an important parameter for monocyte targeting. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to establish the change in thermal properties. The analysis of in-vitro release demonstrated pH-triggered drug cleavage off the delta inulin particles that followed a first-order kinetic process. The efficient targeting ability of the conjugate for RAW 264.7 monocytic cells was supported by cellular uptake studies. Overall, our finding confirmed that semicrystalline delta inulin particles (MPI) can be modified covalently with drugs and such conjugates allow intracellular drug delivery and uptake into monocytes, making this system potentially useful for the treatment of TB. Keywords: inulin; pyrazinoic acid; intracellular delivery; tuberculosis 1. Introduction Mycobacterium tuberculosis (Mtb) remains a deadly and contagious human pathogen responsible for tuberculosis which is now the leading cause of death from infectious diseases surpassing human immunodeficiency virus infection (HIV) and acquired immune deficiency syndrome (AIDS) according to reports from the Global Tuberculosis Report 2017 [1]. Tuberculosis (TB) affects around 2 billion people or 30% of the global population making it a significant public health problem [2]. Despite the progress achieved in global TB control, eradication and treatment has been significantly jeopardized by the HIV epidemic [3–5] and the emergence of multidrug-resistant TB (MDR) and extensively drug-resistant TB (XDR TB) [6,7]. Intracellular pathogens such as Mycobacterium tuberculosis cause chronic infection in human because the pathogen has adapted specialized mechanisms of evading Pharmaceutics 2019, 11, 243; doi:10.3390/pharmaceutics11050243 www.mdpi.com/journal/pharmaceutics Pharmaceutics 2019, 11, 243 2 of 22 and overturning immune recognition, phagosome-lysosome fusion, and destruction by the immune system of the host [8–10]. Also, the pathogen uses infected monocytes as cellular reservoirs [11] and vehicles to spread to other parts of the body [12]. Unfortunately, the Bacille Calmette–Guérin (BCG) vaccine only protects children against disseminated infection but not pulmonary TB and doesn’t provide any protection of adults [13]. Furthermore, current conventional TB treatments are limited in their ability to reach target sites and to fully control both replicating and non-replicating bacteria in infected macrophages making treatment of TB very difficult. Due to its survival strategies the TB pathogen is capable of escaping destruction by therapeutic drugs [13], which translates to treatment failure and extended treatment regimens. Adding pyrazinamide (PZA), which uniquely kills dormant tubercle bacilli, to the drug regimen allows reduction in the duration of TB treatment from 9–12 months to about 6 months. PZA is a prodrug converted intracellularly into pyrazinoic acid (POA) by nicotinamidase enzyme [14]. The use of POA ester derivatives to deliver POA intracellularly has been reported [15,16]. However, the instability and hydrolysis of the POA ester before reaching the target site was a major setback during in-vivo studies despite a promising results in-vitro. Consequently, designing a stable POA prodrug with serum stability and good intracellular targeting remains a major challenge. The host innate immunity plays a key role in protection against Mtb. Monocytes are a highly mobile subclass of the circulating white blood cells that are recruited from the bloodstream to site of infection, injury, and inflammation in the body [17,18]. The recruitment of monocytes is crucial for host defences against invading pathogens, but they also contribute to the pathogenesis of inflammatory diseases [17]. This recruitment, migration, ability to penetrate sites of inflammation, tumours and also cross biological barriers, reaching deep hypoxic areas [19,20]; this makes them an important target that can be utilised to deliver drug cargos in the treatment of conditions such as tuberculosis [21,22], HIV [23,24], cancer [25,26], and inflammatory diseases [27]. Chronic infections like TB could be alleviated by the design of novel drug delivery systems that use the body’s own circulating monocytes to safely deliver drugs intracellularly to the tubercle bacilli reservoir and is an approach that is currently gaining more attention [28]. Delivery of therapeutics to monocytes and macrophages has often exploited particle-based delivery strategies. Modification of the particles surface chemistries, attaching ligands to particles and using different shape and size of particles are different approaches reported in the literature as a strategy in the targeting of drugs to monocytes [29]. A type of particle previously used as a vaccine adjuvant is constructed from inulin, a natural plant fructan found in chicory, dahlia, and Jerusalem artichoke [30]. Inulin consists of linear chains of fructosyl groups linked by β (2 1) glyosidic bonds ! that is terminated at the reducing end by an α-d-(2 1)-glucopyranoside ring [31]. Semicrystalline ! form of inulin (MPI) can be exploited for targeting and sustained drug release to monocytes because this insoluble form of inulin prevents rapid renal excretion. Previous work from our group has shown that this semicrystalline inulin is approximately 1–2 microns in diameter with regular and consistent spherulite-like discoid structure [32]. Due to its potent ability to activate complement, this modified inulin has demonstrated potential use for cancer treatment [33] and as a vaccine adjuvant [34–37]. MPI has excellent tropism towards monocytes, and an ability to bind and be internalized by monocytes with high efficiency [38]. Pharmaceutics 2019, 11, 243 3 of 22 Pharmaceutics 2019, 11, x FOR PEER REVIEW 3 of 21 This study involved the use of MPI as starting material for the development of a novel drug delivery systemThis to target study drugs involved to human the use immune of MPI as cells. starting We hypothesized material for the that development the mononuclear of a novel phagocytes drug delivery system to target drugs to human immune cells. We hypothesized that the mononuclear that act as the principal vehicle for dissemination of tubercle bacilli could also serve as a transporter for phagocytes that act as the principal vehicle for dissemination of tubercle bacilli could also serve as a the delivery of TB drug. In order to improve the stability of the POA conjugates and cellular targeting transporter for the delivery of TB drug. In order to improve the stability of the POA conjugates and of antibiotics to phagocytic cells, we reasoned that by attaching TB drugs to the inulin nanostructure cellular targeting of antibiotics to phagocytic cells, we reasoned that by attaching TB drugs to the surface by using a covalent hydrolysable linkage the conjugate will be cleaved by intracellular acidic inulin nanostructure surface by using a covalent hydrolysable linkage the conjugate will be cleaved conditions and enzymes contained in the lysosome after internalization into monocytes (Figure1). by intracellular acidic conditions and enzymes contained in the lysosome after internalization into Tomonocytes achieve this, (Figure we designed 1). To achieve amine-modified this, we MPIdesigned particles amine-modified coupled to pyrazinoicMPI particles acid coupled (PCA) using to carbodiimidepyrazinoic acid reaction (PCA) chemistry using carbodiimideN-(3-dimethylaminopropyl)- reaction chemistryN0-ethylcarbodiimide N-(3-dimethylaminopropyl)- hydrochlorideN’- (EDC)ethylcarbodiimide and N-hydroxysuccinimide hydrochloride (NHS)(EDC) and that N allows-hydroxysuccinimide pH-triggered and (NHS) controlled that